Flexible Electrophoretic-Type Display

ABSTRACT

An electrophoretic-type display device including an array of microcells, where each microcell is formed by a microwell containing a quantity of ink and a polymer-based membrane connected to upper edges of the microwell. The membrane is formed by curing an aqueous or hydroalcoholic sealing solution that is overcoated on the ink-filled microwells. The ink includes an isoparaffinic-based or oil-based suspension fluid, and the peripheral side walls of the microwell have a surface energy in the range of 20 to 30 mN/m. The microwell material serves two purposes: to prevent displacement (floating) of the relatively light ink solution above the relatively heavy sealing solution, and to facilitate reliable attachment between the polymer membrane and microwell walls during subsequent curing.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/018,185, entitled “Flexible Electrophoretic-Type Display” filed Dec.20, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to non-emissive display devices, andmore particularly to flexible particle-based display devices.

2. Related Art

An electrophoretic display represents one type of particle-based,non-emissive display device that includes an array of several thousandindependently addressable microcells, each microcell containing a smallquantity of electrophoretic ink that is held between a pair of opposed,spaced-apart plate-like electrodes. Each microcell is typically several10s to 100s of microns in size. The electrophoretic ink includes chargedpigment particles suspended in a dyed suspension fluid that ismaintained in an enclosed cell region between the electrodes. At leastone of the two electrodes is transparent so that the state of the inkcan be viewed through the transparent electrode. When a voltagedifference is imposed between the two electrodes, the pigment particlesmigrate to the electrode having a polarity opposite that stored by thepigment particles, thereby facilitating changes in the color displayedthrough the transparent electrode by selectively changing the electrodepolarity. For example, applying a first (e.g., high) voltage potentialto the transparent electrode and an opposite (e.g., low) voltagepotential to the second electrode causes the pigment particles of a1-particle ink to migrate to the transparent electrode, thus producing adisplayed color that is determined by the pigment particles. Reversal ofplate polarity causes the pigment particles to migrate to the secondelectrode, thereby changing the color appearing on the transparentelectrode to that of the dyed suspension fluid. Intermediate colordensity (or shades of grey) may be obtained by varying the electrodepolarity (voltage pulse length and amplitude), thus causing differentnumbers of pigment particles to migrate toward the transparentelectrode. When a 2-particle ink is used, oppositely charged inkparticles of different colors migrate to the opposite electrodesdepending upon the applied polarity. The display may also containelectrophoretic inks with more than two particles as described in U.S.Pat. No. 6,017,584.

Magnetophoretic displays are another type of particle-based display thatutilize a magnetophoretic ink including, for example, dark magnetizableparticles suspended in a solution of white non-magnetic particles. Themagnetizable particles can be, for example, iron or magnetite particles,and the white particles can be titania based particles or otherlight-scattering particles. The suspension fluid may be clear, and anadded surfactant may be added to help maintain a good dispersion. Uponapplication of a magnetic field gradient (e.g. by a magnetic needle) themagnetic particles move in the direction of a higher magnetic fieldstrength. This is the principle of a MagnaDoodle display(http://www.howstuffworks.com/magna-doodle6.htm)).

Other particle-based (emissive) displays may consist of suspendedfluorescent or phosphorescent particles in which the fluorescence orphosphorescence may be activated by, for example, ultraviolet (UV)light.

With the increase in the demand for flexible (e.g., computer) displaysor electric paper, there has arisen a need for particle-based (e.g.,electrophoretic or magnetophoretic) displays in which the particle-basedink (e.g., electrophoretic or magnetophoretic ink) is reliably sealedbetween two flexible substrates. Adjacent microcells of flexibleelectrophoretic displays are typically separated by vertical side wallsto prevent settling and agglomeration of the particles, and serve asspacers between the two opposing electrodes. The height of themicrocells is typically in the range of about 5 microns to about 200microns (particularly in the case of magnetophoretic displays the heightmay be bigger, e.g., in the range of 500 microns-to one millimeter). Itis particularly important for flexible electrophoretic displays toprovide segmented microcells so that the ink cannot move between thecells. Otherwise the moving liquid would destroy the written image uponbending the display. Furthermore, paths between the cells tend to causeparticle concentration gradients.

Conventional methods for producing flexible electrophoretic displaystypically include forming five-sided (open-topped) microwells on aflexible base sheet, inserting a small quantity of electrophoretic inkinto each microwell, and then forming an upper flexible membrane thatattaches to the upper walls of the microwell to seal the electrophoreticink. The key to successfully producing flexible electrophoretic displaysby this method is to form the upper flexible membrane, which is usuallyliquid (viscous) or at least tacky in the uncured form, withoutdisplacing the ink in the open microwells, or causing the adhesive tointeract with the ink in an undesirable manner (e.g., contaminating theink such that agglomeration occurs).

One conventional approach that addresses the problem of formingmembranes without displacing the ink is to utilize a thin layer of arelatively low specific gravity adhesive (or, generally speaking, an“uncured polymer”) that floats on a relatively high specific gravityink. In one version of this method, the ink is inserted in themicrowells, the adhesive is dispensed over the filled microcells, andthen the adhesive is cured to form a membrane. In a second version, theink and adhesive are mixed, the mixture is inserted into the microwellsand allowed to phase-separate (i.e., the adhesive floats to the top ofthe ink), and then the adhesive is cured to form a membrane.

A problem with the conventional methods for forming flexibleelectrophoretic displays is that, by requiring the ink to be relativelyheavy (i.e., to have a relatively high specific gravity), theconventional methods require the use of relatively costly or hazardousink types, such as fluorocarbon or solvent-based inks. That is, thesuccess of these conventional methods depends strongly on the chemicaland physical properties of the ink and the sealing layer compound, andas such require the use of high specific gravity ink solvents that arerelatively hazardous, thus increasing the risk of injury during bothproduction and after-sale use.

What is needed is a method for reliably producing flexibleparticle-based display devices that facilitates the beneficial use oflow-cost, relatively non-hazardous, low specific gravity inks. What isalso needed is a low-cost particle-based display produced by such amethod.

SUMMARY OF THE INVENTION

The present invention is directed to a particle-based (e.g.,electrophoretic or magnetophoretic) display device including an array ofmicrocells containing a particle-based ink having a low specific gravity(e.g., isoparaffinic-based or oil-based) suspension fluid, and sealedwith a polymer-based membrane. To overcome the problems of conventionalelectrophoretic display production methods, the microwell is formed suchthat a surface of the microwell wall (i.e., either the material formingthe wall structure or a surface coating or surface treatment) has asurface energy preferably in the range of 20 to 30 mN/m, and the polymermembrane is formed by overcoating ink-filled microwells with a sealingsolution including a polymer suspended in an aqueous or hydroalcoholicsolvent. By providing the microwell walls with the desired surfaceenergy, the microwell walls are sufficiently attractive to theisoparaffinic/oil-based suspension fluid that they prevent “floating” ofthe ink above the aqueous/hydroalcoholic sealing solution. In addition,microwell walls having this surface energy are sufficiently attractiveto the aqueous/hydroalcoholic sealing solution such that a reliableconnection is established between the polymer and the microwell walls,thereby facilitating the reliable production of high qualityparticle-based display devices.

According to another embodiment of the present invention, a method forproducing particle-based display devices includes forming an array ofmicrowells, substantially filling the microwells with an ink solutionincluding pigment particles suspended in an isoparaffinic/oil-basedsuspension fluid, and then overcoating the filled microwells with asealing solution including a polymer suspended in an aqueous orhydroalcoholic solvent. The use of relatively light ink and relativelyheavy sealing solvent is facilitated by forming or coating/treating themicrowell walls such that the walls are sufficiently compatible withboth the ink and the sealing solution as to both prevent “floating” ofthe ink, and to promote secure attachment of the polymer membrane formedas the aqueous/hydroalcoholic solvent evaporates from the sealingsolution during subsequent curing.

In one specific embodiment, flexible electrophoretic displays areproduced by patterning SU-8 or molding a suitable UV curable polymer toform microwells on a Mylar foil, inserting (e.g., doctorblading orinjecting) an ISOPAR-based electrophoretic ink into the microwells, andthen overcoating the filled microwells with a polymer-based sealingsolution including an ethylester of PVM/MA copolymer in a hydroalcoholicsolvent (e.g., as found in the commercial hair spray V05™).

The invention will be more fully understood in view of the followingdescription of the exemplary embodiments and the drawings thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional side view showing a portion of anelectrophoretic display formed in accordance with an embodiment of thepresent invention;

FIGS. 2(A) and 2(B) are cross-sectional side views showing theelectrophoretic display of FIG. 1 during operation;

FIGS. 3(A), 3(B), 3(C) and 3(D) are simplified cross-sectional viewsillustrating a production method for producing the electrophoreticdisplay of FIG. 1 according to another embodiment of the presentinvention;

FIG. 4 is an SEM image showing molded microwells formed on a flexiblesubstrate in accordance with an embodiment of the present invention;

FIG. 5 is a photograph showing completed microcells formed in accordancewith an embodiment of the present invention;

FIG. 6 is an SEM image showing SU-8 microcells formed in accordance withan embodiment of the present invention; and

FIG. 7 is an SEM image showing SU-8 microcells formed in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION

Unless defined otherwise in this specification, all technical terms areused herein according to their conventional definitions as they arecommonly used and understood by those of ordinary skill in the art. Theterm “microwell” refers to cup-like structures formed in a substantiallyuniform layer of material by photolithographic patterning, molding,microembossing or other manufacturing process. Each microwell thusincludes a lower wall (which may be formed by a substrate on which themicrowell material is deposited) and one or more peripheral side walls(e.g., a single circular wall, or three or more contiguous substantiallystraight walls) that extend upward from the bottom wall and surround apredefined lower wall area, with upper edges of the peripheral sidewalls defining an open end of the microwell. The term “microcell” refersto an image unit formed by a sealed microwell (i.e., a microwellincluding an amount of electrophoretic or other ink and having a sealingmembrane secured to upper edges of the peripheral walls such that theink amount is sealed inside an enclosed space defined by the bottomwall, peripheral side walls, and sealing membrane.

Also, positional terms such as “upper”, “lower”, and “side” are usedbelow and in the appended claims to identify specific structures basedon their relative position to related structures. As such, unlessotherwise defined, these positional terms are not intended to be limitedbased on a fixed external reference.

FIG. 1 is a cross-sectional side view showing a portion of anelectrophoretic display 100 including a microcell 110-1, whichrepresents an exemplary particle-based display device formed inaccordance with the present invention. Microcell 110-1 includes amicrowell 120-1 formed by a portion of an electrically insulatingmaterial layer 120 that is patterned or otherwise processed to form atwo-dimensional lower-wall 122-1 and one or more contiguous peripheralside walls 125-1 that extend upward from lower wall 122-1 and completelysurround a two-dimensional region defined by lower wall 122-1. A width Wof microcell 110-1 (i.e., measured between side walls 125-1) is in therange of 50-1000 microns, and is more preferably in the range of 100-500microns. A height (depth) H of microcell 110-1 (i.e., measured from aninside surface of lower wall 122-1 and upper edges 127-1 of side walls125-1) is in the range of 5 to 200 microns, and more preferably in therange of 10 to 100 microns, with an opening to total area ratio of 0.5to 0.98. Microcell 110-1 also includes a membrane (upper wall) 130-1formed by a portion of a sealing membrane layer 130 that is adhered toupper edges 127-1 of side walls 125-1 and extends over microwell 120-1to form a sealed enclosure defined by bottom wall 122-1, side walls125-1, and membrane 130-1. Located inside this sealed enclosure is anamount of electrophoretic ink (“electrophoretic ink quantity”) 140-1that, in a 1-particle embodiment, includes charged pigment particles 142in a dyed suspension fluid 145 (alternatively, a 2-particle ink mayinclude oppositely charged particles of different colors in a clearsuspension fluid). Electrodes 150-1 and 155-1 are respectively providedunder lower wall 122-1 and over membrane 130-1, and are connected bycontrol circuitry (not shown) to selected voltage sources. At least oneof lower wall 122-1 and membrane portion 130-1 and the associatedelectrode(s) are transparent.

FIGS. 2(A) and 2(B) are cross-sectional side views showing microcell110-1 during operation. When a voltage difference is imposed between theelectrodes 150-1 and 155-1, charged pigment particles 142 migrate towardthe electrode of opposite polarity. For example, as indicated in FIG.2(A), when pigment particles 142 are negatively charged and subject to afield produced by a relatively high (with respect to the potential onelectrode 150-1) positive voltage potential on upper electrode 155-1,pigment particles 142 migrate upward against membrane portion 130-1,thereby causing microcell 110-1 to appear substantially the color ofpigment particles 142 when viewed through membrane portion 130-1.Conversely, when a negative (with respect to the potential on electrode150-1) voltage potential is applied on upper electrode 155-1, pigmentparticles 142 migrate downward against lower wall 122-1, thereby causingmicrocell 110-1 to appear the color of dyed suspension fluid 145 (or, in2-particle inks, the color of the oppositely charged particle) whenviewed through membrane portion 130-1.

According to an embodiment of the present invention, display 100 isformed using a method similar to conventional overcoating methods(mentioned above), but is distinguished in the particular materials usedto form microcell 110-1. In particular, the success of the overcoatingmethod performed in accordance with the present invention is verysensitive to the combined material choices of electrophoretic ink,microwell surface material, and the polymer solution used to form thesealing membrane. It is important that the combination ofelectrophoretic ink, microwell surface material, and sealing solution iswell chosen, otherwise the following effects can occur: a) the ink getspulled out of the microwells during doctorblading of the sealingsolution (this happens, for example, if the surface of the microwellshas low surface energy and the capillary forces on the ink inside themicrowells are weak); b) the sealing solution pulls back and does notwet the surface (this happens for example if the microwell walls arevery hydrophobic or if the ink repels the sealing solution); or c) thesealing solution mixes with the ink (this happens if the ink and thesealing solution are too compatible).

According to an aspect of the present invention, suspension fluid 145 ofelectrophoretic ink 140-1 includes an isoparaffinic-based solvent (e.g.,ISOPAR™) or an oil-based solvent (e.g., a silicone-oil-based solvent),and the peripheral side walls 125-1 of the microwell 120-1 includes awall material or wall coating/treatment such that the surface ofperipheral side walls 125-1 have a surface energy in the range of 20 to30 mN/m (e.g., an epoxy-type, near-UV photoresist such as SU-8). Asdiscussed in additional detail below, this combination facilitates theproduction of microcell 110-1 using a polymer-based sealing solution toform membrane portion 130-1 that includes one of an alkylester of PVM/MAcopolymer (e.g., Poly(methyl vinyl ether-alt-maleic acid monoethylester)(CAS Reg. No. 25087-06-3), Poly(methyl vinyl ether-alt-maleic acidmonobutyl ester) (CAS Reg. No. 25119-68-0), Poly(methyl vinylether-alt-maleic acid monoisopropyl ester) (CAS Reg. No. 31307-95-6)),or PVP-VA (Maleic acid monoisopropyl ester-vinyl methyl ether copolymer,Poly(1-vinylpyrrolidone-co-vinyl acetate copolymer) (CAS Reg. No.25086-89-9 suspended in an aqueous or hydroalcoholic solvent. Thoseskilled in the art will recognize that an electrophoretic ink includingisoparaffinic, oil-based or silicone-oil-based suspension fluid providesan advantage in that it is inexpensive, has low VOC (volatile organiccompounds), and is also preferred for safety reasons (e.g., thesuspension fluids are relatively environmentally friendly, i.e.,non-hazardous) compared to display fluids that rely on fluorocarbons (ofwhich some are expensive and some may damage the ozone layer) orsolvents (which can be harmful to persons when touched or when thevapors are inhaled), as used in conventional methods. Althoughisoparaffinic, oil-based or silicone-oil-based suspension fluids arepresently preferred, suitable suspension fluids may be formed using lessenvironmentally friendly solvents such as toluene and xylenes. Thepresent inventors have also determined that a polymer-based sealingsolution including an ethylester of PVM/MA copolymer suspended in anaqueous or hydroalcoholic solvent produces a superior membrane. Thoseskilled in the art will also recognize that the choice of anaqueous/hydroalcoholic-based sealing solution precludes mixing problem“c” (discussed above) due to the oil-in-water repulsion between thesuspension fluid and the sealing solution solvent. However, although theisoparaffinic/oil-based suspension fluid (ink solvent) will not mix withthe aqueous/hydroalcoholic-based sealing solution, the ink solvent maybecome displaced (i.e., “float”) above the sealing solution due to itslower specific gravity (the lower specific gravity of the suspensionfluid is not a necessary requirement, though). The key, therefore, is toform microwell 120-1 using a wall material that both (1) is attractiveenough to the electrophoretic ink to prevent both problem “a” (discussedabove; i.e., loss of ink during doctorblading) and “floating”displacement of the ink on the sealing solution, and (2) is attractiveenough to the aqueous/hydroalcoholic-based sealing solution tofacilitate reliable attachment of the membrane to the side walls of themicrowell, thus avoiding problem “b” (discussed above; i.e.,delamination of the membrane). The present inventors have determinedthat these beneficial characteristics are achieved when the microwell isformed from a wall material having a surface energy in the range of 20to 30 mN/m, and in one specific embodiment by utilizing SU-8. SU-8 hasbeen determined experimentally to provide both sufficient attraction toisoparaffinic/oil-based electrophoretic ink and sufficient attraction toaqueous/hydroalcoholic-based sealing solution to reliably and repeatablyproduce high quality electrophoretic display devices.

According to a specific embodiment of the present invention, anIsopar-based electrophoretic ink 140-1 was formed by mixing 1 ml IsoparM (Exxon) with 0.15 g TiO₂ (non-chalking), TINT-AYD PC9003, fromElimentis Specialities, Hightstown, N.J. (white pigments), 8 mg SudanRed 7B (red dye), and 14 mg LamChem PE113 from Lambent Technologies asdispersant. The ingredients were ground in an attritor mill to decreasethe particle size, and the resulting ink was tested in a test cell andconsisting of approximately 80 micron high SU-8 cells in a parallelplate configuration. At a voltage potential of approximately 120V theink showed acceptable red/white contrast. SU-8 (from MicroChem, Corp.)cells were filled with this ink and successfully overcoated with asealing solution comprising V05™ hairspray. The present inventors foundthis combination of materials to facilitate a highly repeatable andreliably production process that greatly simplifies the production offlexible electrophoretic displays, thereby overcoming the productionproblems associated with conventional methods.

According to another specific embodiment of the present invention,silicone-based electrophoretic ink was formed by mixing 1 ml Dow Corning200 (50 cst) silicone oil, 0.15 g TiO2 (non-chalking), TINT-AYD PC9003,from Elimentis Specialities, and 4 mg blue dye (e.g., Oil Blue N fromAldrich, Milwaukee, Wis.). The ingredients are ground in an attritormill to decrease the particle size, and some blue/white contrast wasobserved when switching the ink in a similar test cell as the onementioned above. The ink was then successfully sealed in SU-8 cellsaccording to the sealing method described above. The inventors believeDow Corning 510 may be utilized in place of Dow Corning 200.

Other potential pigments, dyes, charge additives and suspending fluidsfor forming electrophoretic inks that can be utilized in accordance withthe present invention are listed in U.S. Pat. No. 6,067,185 and U.S.Pat. No. 5,745,094.

FIGS. 3(A) through 3(D) are simplified cross-sectional side viewsillustrating the basic process steps associated with the production ofmicrocell 110-1 according to another embodiment of the presentinvention.

Referring to FIG. 3(A), the production method begins by forming acup-like microwell structure 120-1 in an insulating material layer 120including lower wall 122-1 wall and one or more peripheral side walls125-1 extending upward from lower wall 122-1. Peripheral side walls125-1 have upper edges 127-1 defining an opening 128 through which anelectrophoretic ink may be inserted into a microcavity 129 defined bylower wall 122-1 and side walls 125-1.

In one specific embodiment, microwell 120-1 is fabricated by depositingan epoxy-type, near-UV photoresist (e.g., SU-8) layer 120 on a flexiblesubstrate (e.g., an Indium Tin-Oxide (ITO) coated Mylar sheet having athickness of 50-250 microns, and then patterning the photoresist usingknown photolithographic techniques to form an array of microwellstructures that are substantially identical to microwell 120-1.Alternatively, the microwell structures may be patterned directly ontoan active matrix backplane, thereby providing the lower electrodedescribed above with reference to FIG. 1.

In an alternative embodiment, microwell 120-1 is molded by transferringa pattern (e.g. in SU-8) into silicone to form a mold, which is thenused to replicate the desired structures in a curable (e.g., UV curable,heat curable, or catalyst assisted curable) polymer layer. Inparticular, Sylgard 184 silicone (Dow Corning Corporation) is used toform the mold, and the UV polymer 60-7165 (urethane acrylate produced byEpoxies, Etc. of Cranston RI) is used to fabricate the replicatedstructures. Curable polymers include, for example, acrylic orepoxy-based photopolymers (i.e., imaging compositions based onpolymers/oligomers/monomers which can be selectively polymerized and/orcrosslinked upon imagewise exposure by light radiation such asultra-violet light) or radiation curable polymers. In one specificembodiment, a mixed composition resin was formed by mixing theepoxy-based polymer 60-7155 and urethane acrylate 60-7165 (both fromEpoxies, Etc. of Cranston). The cured UV polymer layer is flexible andit does not swell in ISOPAR. In one embodiment, microwell 120-1 ismolded onto a thin sheet of Mylar (not shown). In this case the moldingprocess has to be adjusted so that only a thin (a few microns) polymerlayer remains at the bottom to form the lower walls of each microwell.The molded polymer may have to be cleaned (e.g. plasma clean or solventclean) in order to remove silicone residue from the molding process. Asurface coating or surface treatment may also be applied to the moldedmicrowell to produce the desired surface energy.

In addition to photolithograpy and molding, several other alternativefabrication methods may be utilized to produce microwell 120-1. Forexample, microwell 120-1 may also be formed by microembossing or a knownmicrofabrication method (e.g. etching, laser ablation, etc.). Moreover,in addition to using photoresist and UV polymers, other polymer systems(e.g., two-component systems) may also be used to form microwell 120-1.

Referring to FIG. 3(B), electrophoretic ink quantity 140-1 is theninserted into microwell 120-1 through opening 128. As discussed above,electrophoretic ink quantity 140-1 includes electrophoretic (charged)pigment particles 142 dispersed in a suspension fluid 145 (i.e., eitheran isoparaffinic-based solvent such as ISOPAR, or an oil-based solventor silicone oil) having a surface tension in the range of 18 to 30 mN/m.Other surface tension values may be possible if the values for the wallsand the sealing solution change accordingly. The viscosity of thesuspending fluid may also affect how broad this range can be. Theinsertion process is preferably performed such that the cells areslightly underfilled (i.e., such that upper edge 127-1 of side walls125-1 extends above an upper surface 146 of electrophoretic ink quantity140-1. In alternative embodiments, the insertion process is performedusing doctorblading or injection by inkjet printing or spray coating andevaporating the excess ink.

FIG. 3(C) depicts overcoating microwell 120-1 with a sealing solutionlayer 130 including a polymer suspended in one of an aqueous solutionand a hydroalcoholic solvent. In one embodiment, sealing solution layer130 is overcoated by doctorblading or curtain coating sealing solutiononto upper edge 127-1 of side walls 125-1 such that a lower surface 136of sealing solution layer 130 contacts upper surface 146 ofelectrophoretic ink solution 140-1. In a currently preferred embodiment,sealing solution layer 130 includes an aqueous/hydroalcoholic solventcontaining an ethylester of PVM/MA copolymer (i.e., a film formingpolymer: monoethyl ester of poly (methyl vinyl ether/maleic anhydride))having a surface tension of about 100-140 mN/m. Sealing solution layer130 may include several additional additives including (but not limitedto) aminomethylpropanol (makes the film more resistant to humidity),triethyl citrate (plasticizer, and reduces tackiness), and dimethiconecopolyol (surfactant and reduces tackiness). In one specific embodimentthe aqueous/hydroalcoholic solvent includes 40% ethanol in water.Sealing solution layer 130 may also contain a dye or pigments to changethe display color (in case the sealed side is the viewed side), and maycontain nanoparticles to increase or lower its refractive index (e.g.,if index matching to a glass substrate is desirable). Moreover,conductive materials such as carbon nanotubes and Baytron conductivepolymer may be added (this may be particularly useful in magnetophoreticdisplays to dissipate static charge). As discussed above, by formingmicrowell 120-1 using a material having a surface energy in the range of20 to 30 mN/m and by forming microwell 120-1 such that it has a width inthe range of 50 to 1000 microns, the attraction between electrophoreticink solution 140-1 and peripheral side walls 125-1 produces a force onelectrophoretic ink solution 140-1 that prevents upward displacement(floating) of the lighter electrophoretic ink solution 140-1 abovesealing solution layer 130. Accordingly, sealing solution layer 130 isstably and reliably supported by electrophoretic ink solution 140-1 andupper edges 127-1 of side walls 125-1 in an ideal position to form apolymer membrane.

FIG. 3(D) depicts curing of the sealing solution (indicated by verticalwavy lines) such that the aqueous/hydroalcoholic solvent is evaporatedfrom the sealing solution, thereby causing the remaining polymer to formmembrane 130-1 that covers the microwell opening, thereby sealingelectrophoretic ink solution 140-1 inside a chamber formed by microwell120-1. The sample may be heated to an elevated temperature (e.g., up to90° C.) in order to achieve more complete evaporation of theaqueous/hydroalcoholic solvent). As discussed above, by formingmicrowell 120-1 using a material having a surface energy in the range of20 to 30 mN/m, the attraction between sealing solution layer 130 andperipheral side walls 125-1 produces a secure connection betweenmembrane 130-1 and upper edges 127-1 of side walls 125-1 (however, ingeneral the surface energy values may be much broader, e.g. by addingadequate surfactants to the aqueous/hydroalcoholic solution we may beable to broaden the ranges).

Membrane 130-1 can be made more robust by coating another material overmembrane 130-1 after the curing process. In one embodiment, aphotosensitive polymer (Dymax 401) is coated on membrane 130-1, and thencross-linked with UV light. A very smooth surface is formed by pressingthe polymer flat with a Mylar sheet. After UV exposure the Mylar ispeeled off, which is possible due to its poor adhesion to the polymer.

The present inventors found that a commercially available liquidhairspray (V05™, produced by Alberto-Culver USA, Inc.) can be used asthe sealing solution used to seal the ink-filled microcells describedabove. Like the preferred embodiment, this hairspray is based onethylester of PVM/MA copolymer, and includes aminomethylpropanol,triethyl citrate, and dimethicone copolyol, along with fragrance and adiisopropyl adipate emollient. The hairspray, which comes in form of aliquid, was simply spread over the filled microcells with a doctorblademade from a sheet of Mylar foil. Afterwards, the layer was cured (thesolvent was evaporated off) with a warm air gun. The inventors foundthat the resulting membrane became brittle and started to crack after afew days (i.e., after it had dried completely). For this reason PEG20,000(polyethylene glycol) was added at approximately 2-4% (wt) to thesealing solution as a plasticizer with good results. PEG also seems toreduce the surface tension of the sealing solution layer, preventing itfrom beading up in some cases. It was also found that an increasedviscosity of the sealing solution produced better results concerningpull-back/de-wetting (the de-wetting is much less pronounced). A higherviscosity was achieved by evaporating off some of the solvent. Reducingthe liquid volume to 40% of the original volume resulted in a quiteuseful viscosity. A higher viscosity of the sealing solution alsoproduced a thicker membrane. Thicker membrane layers are also achievedby applying several layers of the polymer. The adhesion of this materialto the SU-8 cell walls was very good. In delamination tests the SU-8walls lifted off the ITO substrate instead.

According to another specific embodiment, the sealing solution layerincludes an aqueous/alcoholic carrier (hydroalcoholic solution) and asilicone component. One component in the hairspray-based sealing polymeris silicone. It is known that silicones reduce adhesion (e.g., they areoften used in de-molding applications). The inventors have observed inexperiments that the particle adhesion seems to be lower when siliconeis present in the surface layer. This means that the contrast can behigher and the display may switch faster (the inventors observed fasterswitching speed and a better dark state in a one-particle ink system).However, silicone by itself swells when in contact with ISOPAR, andsilicone oil may leach out into the ink, which changes the ink'sbehavior. Many silicones are usually also not compatible with aqueoussolutions. However, there is a group of hydrophilic (organomodifiedsilicones) that are compatible with aqueous systems. Theseorganomodified silicones have slight to complete solubility in water andare composed of dimethylsiloxane molecular backbones in which some ofthe methyl groups are replaced by polyalkylenoxy groups linked through apropyl group to the silicon atom (e.g., DBE or DBP polymers from Gelest,Inc.). These organomodified silicones are also calleddimethiconecopolyols (see, for example, U.S. Pat. No. 6,703,026, column5). Other examples of dimethiconecopolyols are BC403 from BasildonChemicals or Ultrasil™ dimethicone copolyol blends (blend withpolyethylene glycol) from Noveon, Inc.

FIGS. 4 through 7 are pictures of electrophoretic media fabricated inaccordance with the present invention. FIG. 4 is a Scanning ElectronMicroscopy (SEM) image showing molded microwells formed on a Mylar foil.FIG. 5 is a photograph showing completed microcells with SU-8 walls. Themicrocells are approximately 200 microns wide and 50 microns deep. FIG.6 is an SEM image of sealed microcells, and shows a thin membrane withexcellent adhesion to the SU-8 walls (no delamination is visible afterbreaking the sample in half). FIG. 7 is an SEM image of a sealed sample(filled with purple electrophoretic ink). The sample was broken in halfand ink has evaporated from the broken cells. Notice that there islittle particle adhesion visible on the membrane; it appears rathertransparent.

In accordance with another specific embodiment of the present invention,a magnetophoretic display was produced by incorporating amagnetophoretic ink in a display structure such as that described above.The magnetophoretic ink was produced by mixing 1 ml Isopar M, 14 mgLamChem PE113 from Lambent Technologies as dispersant, 100 mg whiteTi-pure R700 TiO₂ (titania) particles from Dupont, and 26 mg Iron(II,III) oxide from Alfa Aesar. The mixture was agitated by ultrasonicagitation for approximately five minutes. The inventors observed adark/white contrast when a magnet is brought in close proximity with theink (the Iron oxide particles were magnetized by the magnetic field andattracted towards the magnet; the white non-magnetic pigments are beingdisplaced by the dark magnetic particles which causes a contrast). TheIsopar-based magnetophoretic ink was then successfully sealed in SU-8cells using the sealing method described above.

In addition to the specific embodiments described above, othercombinations of the features associated with the present invention maybe advantageously combined. For example, the sealing method describedherein may be useful for sealing other liquids (non-aqueous,non-alcoholic) such as liquid crystals, oils, etc. In addition, althoughthe present invention is described above with specific reference toelectrophoretic and magnetophoretic particle-based displays, the variousaspects and features of the present invention may be utilized to formdisplay devices using ink solutions containing other particle types(e.g., fluorescent or phosphorescent) and may also be utilized tocontain liquids in non-display devices. Accordingly, unless otherwisespecified, the appended claims should not necessarily be interpreted asdirected to electrophoretic and magnetophoretic displays.

1. A display device including a plurality of microcells, each microcellcomprising: a microwell including a lower wall and one or moreperipheral side walls extending upward from the lower wall; an inksolution contained within the microwell, the ink solution including asuspension fluid; and a membrane attached to upper edges of theperipheral side walls such that the ink solution is sealed inside of achamber defined by the membrane and the lower wall and peripheral sidewalls of the microwell, wherein the membrane comprises at least one ofan alkylester of PVM/MA copolymer, a PVP/VA copolymer, and a siliconecopolyol.
 2. The display device according to claim 1, wherein theperipheral side walls of the microwell comprise a surface energy in therange of 20 to 30 mN/m.
 3. The display device according to claim 1,wherein the microwell has a width in the range of 10² to 10³ microns,and a height in the range of 5 to 1000 microns.
 4. The display deviceaccording to claim 1, wherein the wall material comprises one of acurable polymer and an epoxy-type, near-UV photoresist.
 5. The displaydevice according to claim 1, wherein the suspension fluid comprises oneof a silicone-based oil and ISOPAR.
 6. The display device according toclaim 1, wherein the ink solution comprises one of an electrophoreticink and a magnetophoretic ink.
 7. A display device including a pluralityof microcells, each microcell comprising: a microwell including a lowerwall and one or more peripheral side walls extending upward from thelower wall; an ink solution contained within the microwell, the inksolution including a plurality of particles in a suspension fluid; and apolymer-based membrane attached to upper edges of the peripheral sidewalls such that the ink solution is sealed inside of a chamber definedby the membrane and the lower wall and peripheral side walls of themicrowell, wherein the suspension fluid comprises one of anisoparaffinic-based solvent and a silicone oil-based solvent, andwherein the membrane comprises at least one of an alkylester of PVM/MAcopolymer, a PVP/VA copolymer, and a silicone copolyol.
 8. The displaydevice according to claim 7, wherein the peripheral side walls of themicrowell comprise a surface energy in the range of 20 to 30 mN/m. 9.The display device according to claim 7, wherein the microwell has awidth in the range of 10² to 10³ microns, and a height in the range of 5to 1000 microns.
 10. The display device according to claim 7, whereinthe wall material comprises one of a curable polymer and an epoxy-type,near-UV photoresist.
 11. The display device according to claim 7,wherein the suspension fluid comprises one of a silicone-based oil andISOPAR.
 12. The display device according to claim 7, wherein the inksolution comprises one of an electrophoretic ink and a magnetophoreticink.
 13. A display device including a plurality of microcells, eachmicrocell comprising: a microwell including a lower wall and one or moreperipheral side walls extending upward from the lower wall; one of anelectrophoretic ink and a magnetophoretic ink contained within themicrowell; and a membrane attached to upper edges of the peripheral sidewalls such that said one of said electrophoretic ink and amagnetophoretic ink is sealed inside of a chamber defined by themembrane and the lower wall and peripheral side walls of the microwell,wherein the membrane comprises at least one of an alkylester of PVM/MAcopolymer, a PVP/VA copolymer, and a silicone copolyol.
 14. The displaydevice according to claim 13, wherein the peripheral side walls of themicrowell comprise a surface energy in the range of 20 to 30 mN/m. 15.The display device according to claim 13, wherein the microwell has awidth in the range of 10² to 10³ microns, and a height in the range of 5to 1000 microns.
 16. The display device according to claim 13, whereinthe wall material comprises one of a curable polymer and an epoxy-type,near-UV photoresist.
 17. The display device according to claim 13,wherein said one of said electrophoretic ink and a magnetophoretic inkcomprises one of a silicone-based oil and ISOPAR.